I have added two columns to a document library under online version. First worked fine to start with. Now second is a required field, but cant update the library either as individual items or as a multiple items. Field change is "saved" but then reverts back to previous state.
How did you change the column value? Right click the item and click properties?
Narrow down this issue as following:
Create a new multiple choice column with required and have a test to check the result.
Besides, you could use Quick edit to change the value and compare the result. Note: set Allow 'Fill-in' choices to NO if use quick edit.
And check if there is workflow or event receiver to update the column with previous value.
If the issue exists, please create a new library and column to check the result. And also you could clear the browser cache and change a browser to check the result.
If necessary, you can reboot or change the computer to test it.